21 weeks might be a wee bit early, but a cull is a cull and you could certainly go ahead and process cull males. They'd be just about right for meat age, in my experience, as well.
I'd maybe recommend this. Start "culling" mentally right now. You already know a male here and a male there that simply aren't going to make grade. Go ahead and put a brightly colored, loose fitting zip tie on them or better, another leg band on the second leg as well. Or, another method is band on left means a loser while a band on right means a keeper, at least for now.
Another thing I noticed? Are your pullets and cockerels running together? I really like to separate out the males and females as early as I can judge their sex. Each sex will do better, but especially the pullets will do better if no males are in their pen competing for feed and space.
In my experience, it also just helps your eye seeing males with males. The cream rises to the top, visually. The lead balloons sink to the bottom visually. Cull off the bottom, as it were. Keep choosing "meat birds" off the bottom and tagging them until you're left with 3 or 4 finalist K's. Be in no rush to judgement concerning those 3 or 4 finalists. You have approximately 5 weeks before the axe will fall. By then, I suspect you'll be quite able to tag your "heir" and your "spare".
The birds show how well you've fed them and cared for them. They look extra-ordinarily nice.
edit to add: Male 21 is already a finalist. He may not make the very, very final cut, but he's not a cull at this time. Photos are very hard to capture the true nature of a bird. Nonetheless. unless #17 jumps into gear, and quickly, he's gonna taste good with noodles. The rest? time will tell.